U.S. patent application number 10/558412 was filed with the patent office on 2007-02-15 for electrochemical oxygen separator cell.
Invention is credited to Vicenzo Antonucci, Antonino Salvatore Arico, Xicola Agustin Sin, Antonio Zaopo.
Application Number | 20070034507 10/558412 |
Document ID | / |
Family ID | 33483757 |
Filed Date | 2007-02-15 |
United States Patent
Application |
20070034507 |
Kind Code |
A1 |
Sin; Xicola Agustin ; et
al. |
February 15, 2007 |
Electrochemical oxygen separator cell
Abstract
An electrochemical oxygen separator cell including an electrode
based on lanthanum strontium manganese oxide or lanthanum strontium
cobalt iron oxide; and an electrolyte membrane of doped ceria.
Inventors: |
Sin; Xicola Agustin;
(Milano, IT) ; Zaopo; Antonio; (Milano, IT)
; Antonucci; Vicenzo; (Messina, IT) ; Arico;
Antonino Salvatore; (Contesse, IT) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Family ID: |
33483757 |
Appl. No.: |
10/558412 |
Filed: |
May 28, 2003 |
PCT Filed: |
May 28, 2003 |
PCT NO: |
PCT/EP03/05633 |
371 Date: |
September 7, 2006 |
Current U.S.
Class: |
204/290.1 ;
204/295 |
Current CPC
Class: |
B01D 53/326
20130101 |
Class at
Publication: |
204/290.1 ;
204/295 |
International
Class: |
C25C 7/04 20060101
C25C007/04; C25B 13/00 20060101 C25B013/00 |
Claims
1-12. (canceled)
13. An electrochemical oxygen separator cell comprising: a cathode
comprising a material selected from lanthanum strontium manganese
oxide/doped ceria in a ratio of 85:15 to 75:25 by weight; and
lanthanum strontium cobalt iron oxide; an electrolyte membrane
comprising ceria doped from 15 to 25% by mole; and an anode
comprising a material selected from lanthanum strontium manganese
oxide/doped ceria in a ratio of about 85:15 to about 75:25 by
weight; and lanthanum strontium cobalt iron oxide.
14. The electrochemical oxygen separator cell according to claim
13, wherein the ceria is doped with an oxide selected from
gadolinia and samaria.
15. The electrochemical oxygen separator cell according to claim
13, wherein the ceria is doped at 20% by mole.
16. The electrochemical oxygen separator cell according to claim
13, wherein the doped ceria of the electrolyte membrane is
Ce.sub.0.8Gd.sub.0.2O.sub.1.90.
17. The electrochemical oxygen separator cell according to claim
13, wherein the lanthanum strontium manganese oxide/doped ceria
ratio is 80:20 to 70:30 by weight.
18. The electrochemical oxygen separator cell according to claim
13, wherein the lanthanum strontium manganese oxide is
La.sub.0.8Sr.sub.0.2MnO.sub.3.
19. The electrochemical oxygen separator cell according to claim
13, wherein the cathode or anode or both comprises lanthanum
strontium cobalt iron oxide.
20. The electrochemical oxygen separator cell according to claim
19, wherein the lanthanum strontium cobalt iron oxide
La.sub.0.6Sr.sub.0.4Fe.sub.0.8Co.sub.0.2O.sub.3.
21. The electrochemical oxygen separator cell according to claim
13, wherein the lanthanum strontium cobalt iron oxide is added with
doped ceria.
22. The electrochemical oxygen separator cell according to claim
13, wherein at least one of the cathode and the anode is thicker
than the electrolyte membrane.
23. The electrochemical oxygen separator cell according to claim
22, wherein the anode is thicker than the electrolyte membrane.
24. An apparatus comprising an electrochemical oxygen separator
cell as described in any one of claims 13-23.
Description
[0001] The present invention relates to electrochemical cells for
separating oxygen from other gases to produce an oxygen-enriched
stream or an oxygen-depleted stream.
[0002] A large number of commercial processes needs oxygen,
oxygen-enriched or oxygen-depleted stream. Examples of industrial
processes requiring oxygen enriched stream uses include glass
production, petrochemical industry, paper industry, metallurgical
industry, aerospace and medical applications. Oxygen depleted
streams can be advantageous for lowering the emission of nitrogen
oxides (NO.sub.x) by diesel engines.
[0003] There are many differing methods and apparatus used for
separating oxygen from other fluids, such as cryogenic cycles,
non-cryogenic air separation plants, including the use of molecular
sieves.
[0004] One of such methods uses an electrochemical process where an
oxygen-containing gaseous mixture, such as air, is fed to one side
of a ceramic membrane with an electrical potential applied across
the membrane. The oxygen molecules are reduced to oxygen ions at
the interface between the cathode and the electrolyte membrane and
the oxygen ions can selectively pass through the electrolyte.
[0005] After passing through the electrolyte, a further reaction
takes place at the interface between the electrolyte and the anode
where the oxygen ions are oxidized to reform oxygen molecules. By
using of particular electrolyte membranes, only the oxygen ions are
allowed to pass through the cell and thus the overall process is
very selective for producing a stream with high concentration of
oxygen.
[0006] In further detail, when an electrical potential is applied
across an oxygen ion electrolyte membrane via electrodes, oxygen is
dissociated and reduced at the cathode according to the following
reaction O.sub.2.fwdarw.4e.sup.-.fwdarw.2O.sup.-2
[0007] Oxygen ions migrate through the electrolyte, and are
oxidised and recombined at the anode to produce oxygen. An external
electrical connection allows the transfer of electrons from the
anode to the cathode. The flux of oxygen produced by an
electrically driven force is directly proportional to the current
passing through the electrolyte membrane according to the Faraday
law, mols .times. .times. O 2 = I .times. t 4 .times. F ##EQU1##
wherein I is the electrical current (A), [0008] t is time (sec),
[0009] F is the Faraday constant (i.e. 96485.3 C/eq) and [0010] 4
is the number of electrons exchanged in the electrochemical
reaction 2O.sup.-2.fwdarw.4e.sup.-+O.sub.2, eq/mol.
[0011] This means that the flux of oxygen for an applied potential
is governed by the electrochemical resistance of the cell (the sum
of the electrolyte and electrode polarisation resistance). The
O.sub.2 flux can be increased by either raising the potential of
the electrochemical cell or reducing the resistance of the
membrane.
[0012] U.S. Pat. No. 5,021,137 (in the name of Ceramatec Inc.)
relates to a ceramic solid electrolyte based electrochemical oxygen
concentrator cell. The cell is based on a doped cerium oxide
ceramic solid electrolyte and lanthanum strontium cobaltite (LSCO)
ceramic electrodes. Preferably, cerium oxide is doped with calcium
oxide, strontium oxide or yttrium oxide. This cell exhibits a
current density of 450 mA/cm.sup.2 at 800.degree. C. and 1.0V dc
operating voltage.
[0013] D. Waller et al. Steele, Electrochemical Society Proceedings
Vol. 95-24 (1997) pp. 48-64 disclose oxygen separation using dense
gadolinia doped ceria membranes. More specifically,
Ce.sub.0.9Gd.sub.0.1O.sub.1.95 (ceria doped with 10% of gadolinia,
hereinafter referred to as CGO-10) as electrolyte is screen printed
or painted with lanthanum strontium cobalt iron oxide (LSFCO) as
electrodes. The X-ray diffraction (XRD) data of the LSFCO
electrodes show a polyphase pattern. The current density provided
by this construction is of almost 350 mA/cm.sup.2 at 800.degree. C.
and 0.6V dc operating voltage.
[0014] This paper reports that for achieving a production of 1 ml
of oxygen per minute and per cm.sup.2, it is necessary a separator
showing a current density of at least 287 mA/cm.sup.2. It is
established that the electrode resistance is the predominant factor
in limiting the oxygen flux through the cell. Reducing the
electrode resistance is the key factor in increasing the
performance of the cell. The electrolyte resistance constitutes
only a small proportion of the overall resistance of the cell;
therefore the thickness of the electrolyte may be increased (e.g.
from 100 to 250 .mu.m) to improve the mechanical strength of the
cell without giving rise to a large increase in the overall
resistance of the cell.
[0015] Applicant faced the problem of providing an electrochemical
oxygen separator cell with higher performance, in term of current
density and, as a consequence, of oxygen separation, with respect
to those known in the art.
[0016] This problem is solved by providing an electrochemical
oxygen separator cell with a specific combination of material for
electrolyte membrane and electrodes which yields surprisingly high
performances also in the presence of a cell architecture wherein
the supporting element is one of the electrode, thus having a
thickness greater than that of the electrolyte membrane.
[0017] Therefore the present invention relates to an
electrochemical oxygen separator cell including [0018] a cathode
comprising a material selected from lanthanum strontium manganese
oxide/doped ceria in a ratio ranging between about 85:15 and about
75:25 by weight; and lanthanum strontium cobalt iron oxide; [0019]
an electrolyte membrane comprising ceria doped from about 15 to
about 25% by mole; [0020] an anode comprising a material selected
from lanthanum strontium manganese oxide/doped ceria in a ratio
ranging between about 85:15 and about 75:25 by weight; and
lanthanum strontium cobalt iron oxide.
[0021] Unless otherwise indicated, in the following of the
description lanthanum strontium cobalt iron oxide will be referred
to as LSFCO, and lanthanum strontium manganese oxide will be
referred to as LSMO.
[0022] In the following of the description, cathode and anode could
also be referred to as "electrode".
[0023] Examples of doped ceria useful in the present invention are
gadolinia doped ceria and samaria doped ceria.
[0024] The doped ceria is used as electrolyte membrane material is
preferably doped in an amount of about 20% by mole. Preferred in
this connection is Ce.sub.0.8Gd.sub.0.2O.sub.1.90 (hereinafter
referred to as CGO-20).
[0025] Cathode and anode of the present invention can have the same
or different composition and morphology.
[0026] Preferably, lanthanum strontium manganese oxide/doped ceria
ratio is from about 80:20 to about 70:30 by weight.
[0027] Preferably La.sub.1-xSr.sub.xMnO.sub.3-.delta. is
La.sub.0.8Sr.sub.0.2MnO.sub.3 (hereinafter referred to as
LSMO-80).
[0028] Preferred material for electrode is
La.sub.1-xSr.sub.xFe.sub.1-yCo.sub.yO.sub.3-.delta., wherein x and
y are independently equal to a value comprised between 0 and 1
included and 8 is from stoichiometry, more preferably
La.sub.0.6Sr.sub.0.4Fe.sub.0.8Co.sub.0.2O.sub.3-.delta.
(hereinafter referred to as LSFCO-80). Preferably, LSFCO is in
single phase belonging to the perovskite family, i.e. a group of
compounds of the general formula ABX.sub.3 with X most frequently
oxygen. Optionally, LSFCO can be added with doped ceria.
[0029] Both cathode and anode preferably show a porosity at least
of about 20% (measured by SEM).
[0030] In a preferred embodiment, electrochemical oxygen separator
cell of the invention shows one electrode (supporting electrode)
being substantially thicker than the electrolyte membrane.
Preferably the supporting electrode is the anode.
[0031] For example, the supporting electrode shows a thickness of
about 100-600 .mu.m. For example, the electrolyte membrane has a
thickness ranging between about 0.5 .mu.m and about 20 .mu.m, more
preferably between about 2 .mu.m and about 10 .mu.m.
[0032] In another aspect the present invention relates to an
apparatus comprising the electrochemical oxygen separator cell
disclosed above. Said apparatus may be an engine for vehicle
transportation, electrochemical reactors for synthesis of syn-gas
from hydrocarbons, alcohols, acids from methane, high purity oxygen
supplier for medical applications and for petrochemical, aerospace
and metallurgy industries.
[0033] One of the applications for the electrochemical oxygen
separator cell of the invention is related to the reduction of the
contaminant emission in diesel engine, in particular NO.sub.x
exhausts and particulates. In this connection, the electrochemical
oxygen separator cell finds application in two ways, i.e. enriching
or depleting the oxygen flow to the engine. The enriching of oxygen
flow to the engine reduces particulate emissions, especially at
cold start, increases engine power output, and allows the use of
lowergrade fuels.
[0034] The depleting of oxygen flow to the engine reduces NOx
emissions without the problems caused by exhaust gas re-circulation
(engine wear, oil contamination), and eliminates the need for a
heat exchanger to cool exhaust gases before recirculation.
[0035] The present invention will be further illustrated by means
of the following examples and figures, wherein:
[0036] FIG. 1 illustrates an XRD of the CGO powders as prepared in
example 1 treated at different temperatures;
[0037] FIG. 2 schematically shows the polarization experimental
setup;
[0038] FIG. 3 illustrates the polarization measurement of the
electrochemical oxygen separator cell of Example 1;
[0039] FIG. 4 illustrates the polarization measurement of the
electrochemical oxygen separator cell of Example 2;
[0040] FIG. 5 illustrates the polarization measurement of the
electrochemical oxygen separator cell of Example 3;
[0041] FIG. 6 illustrates the polarization measurement of the
electrochemical oxygen separator cell of Example 4;
[0042] FIG. 7 illustrates the polarization measurement of the
electrochemical oxygen separator cell of Example 5;
[0043] FIG. 8 illustrates the polarization measurement of the
electrochemical oxygen separator cell of Example 6.
[0044] An electrochemical oxygen separator cell according to the
schematic drawing of FIG. 2 comprises an anode (1), an electrolyte
membrane (2), a cathode (3), and metal contacts (4) for the
connection to electric circuits.
EXAMPLE 1
LSMO/CGO-CGO-LSMO/CGO (Symmetric)
[0045] An electrochemical oxygen separator cell with the following
structure and composition was prepared and tested:
Cathode: Composition: 30% wt. of CGO-20+70% wt of LSMO-80
[0046] Thickness: .about.20 .mu.m. Electrolyte membrane:
Composition: CGO-20 [0047] Thickness: 450 .mu.m Anode: Composition:
30% wt. of CGO-20+70% wt. of LSMO-80 [0048] Thickness: .about.20
.mu.m. a) CGO-20 Powder Synthesis
[0049] A solution of 12.6 g pf oxalic acid (Aldrich 99.999%) in 250
ml of H.sub.2O was brought to pH=6.5 with NaOH (0.1M) (Aldrich).
8.0 g. of Ce(NO.sub.3).sub.3.6H.sub.2O (Aldrich 99.99%) and 2.078 g
Gd(NO.sub.3).sub.3.6H.sub.2O (Aldrich 99.99%) were added to 50 ml
of H.sub.2O and stirred up to complete dissolution. This cationic
solution was dropwise added to the oxalic solution to give a ratio
1 mol Ce.sup.3+:.about.6 mol H.sub.2C.sub.2O.sub.4 and 1 mol
Gd.sup.3+:.about.6 mol H.sub.2C.sub.2O.sub.4. The formed
precipitate was filtered, thrice washed with water and dried at
100.degree. C. for 4 hours. The pH of the water used for washing
was up to 6.5. The dried powder was crashed and crystallised at
700.degree. C. for 4 h. A CGO-20 nanopowder (4 g) was obtained. The
nanopowder has a particle size of 26 nm measured from the XRD
pattern (FIG. 1) by line broadening measurements using the Scherrer
equation Kl/.beta.Cos .theta.
[0050] wherein K is the shape factor of the average
crystallite;
[0051] l is the wavelength,
[0052] .beta. (rad) is the full width at half maximum of an
individual peak, and
[0053] .theta. (rad) is the peak position (2.theta./2).
b) CGO-20 Electrolyte Membrane Preparation.
[0054] CGO-20 powder of point a) was thermally treated at
1050.degree. C. for 1 h, then uniaxially pressed at 300 MPa, and
the resulting pellet was thermally treated at 1550.degree. C. for 3
hours to give a membrane about 450 .mu.m thick, with a relative
density (experimental density/theoretical density) higher than 95%.
c) Electrode Preparation [0055] 30 mg of CGO powders of point a)
were mixed in an agata mortar with 70 mg of LSMO-80 (Praxair
99.9%), and added with 1.5 ml of isopropanol (Carlo Erba) in a
ultrasonic bath for 10 min to give a slurry. Said slurry was
painted on both side of the electrolyte membrane of point b), then
dried at 150.degree. C. for 1 h in air conditions. The electrode
and electrode/membrane interface were sintered at 1110.degree. C.
for 2 h in air conditions. d) Polarisation Measurement.
[0056] The polarisation measurement was carried out by
potentiodynamic measurement [by applying a constant voltage (V) and
measuring the current (I)] in a four electrode cell-configuration
(FIG. 2). The measurements were carried out by an AUTOLAB Ecochemie
potentiostat/galvanostat and impedance analyzer, at 800.degree. C.
The results are set forth in FIG. 3. A current density of 1.0
A/cm.sup.2 was observed at 1.4V do operating voltage.
EXAMPLE 2
LSCFO-CGO-LSCFO (Symmetric)
[0057] An electrochemical oxygen separator cell with the following
structure and composition was prepared and tested:
Cathode: Composition: LSCFO-80
[0058] Thickness: .about.20 .mu.m. Electrolyte membrane:
Composition: CGO-20 [0059] Thickness: 450 .mu.m Anode: Composition:
LSCFO-80 [0060] Thickness: .about.20 .mu.m.
[0061] CGO-20 powder and membrane were prepared as from steps a-b)
of Example 1.
a) Electrode Preparation
[0062] LSFCO-80 powder (100 mg; single perovskite phase, primary
particle size 9 nm, BET surface area: 4.12 m.sup.2/g, Praxair) was
homogenised in an agata mortar (with 1.5 ml of isopropanol (Carlo
Erba) in an ultrasonic bath for 10 min, to provide a slurry. Both
sides of the CGO-20 electrolyte membrane were painted with said
slurrys and dried at 150.degree. C. for 1 hour in air conditions,
then the electrode and electrode/membrane interface were sintered
at 1110.degree. C. for 2 hours in air conditions. b) Polarisation
Measurement.
[0063] The cell evaluation was carried out as described in Example
1, d). The results are set forth in FIG. 4. A current density of
1.2 A/cm.sup.2 was observed at 0.6V dc operating voltage.
EXAMPLE 3
LSMO/CGO-CGO-LSMO/CGO (Asymmetric)
[0064] An electrochemical oxygen separator cell with the following
structure and composition was prepared and tested:
Cathode: Composition: 30% wt. of CGO-20+70% wt of LSMO-80
[0065] Thickness: .about.20 .mu.m. Electrolyte membrane:
Composition: CGO-20 [0066] Thickness: 8 .mu.m Anode: Composition:
30% wt. of CGO-20+70% wt. of LSMO-80 [0067] Thickness: .about.500
.mu.m.
[0068] CGO-20 powder was prepared as from step a) of Example 1.
a) Anode Preparation
[0069] 0.21 g of CGO-20 and 0.49 g of LSMO-80 (Praxair) were mixed
in an agata mortar. The mixed powders were are pressed in a
cylindrical shape (.phi.=16 mm, d=1 mm) under uniaxial pressure of
200 MPa for 30 min. The resulting green pellet was sintered at
900.degree. C. for 2 hours. b) Electrolyte Membrane Preparation
[0070] CGO-20 powder (1 g) was mixed with ethanol (2 ml) in a ball
mill for 4 hours to give a slurry. Said slurry (0.5 g 20 ml of
ethanol, and the resulting suspension was placed for 4 hours in an
ultrasonic bath. The resulting solution was sprayed by an aerograph
device on the anode (supporting electrode) of step a) for 3 min,
then sintered at 1300.degree. C. for 6 hours. c) Cathode
Preparation [0071] 0.21 g of CGO-20, made in example 1, a) were
mixed in agata mortar with 0.49 g of LSMO-80 (Praxair 99.9%), by
adding 2 ml of ethanol, for 4 hours, to give a slurry. Said slurry
(0.5 g) was added with 20 ml of ethanol, and the resulting
suspension was placed for 4 hours in an ultrasonic bath. The
resulting solution was sprayed by an aerograph device on the
electrolyte membrane of step b) supported by the anode, for 3 min,
then sintered at 1100.degree. C. for 1 hour. d) Polarisation
Measurement.
[0072] The cell evaluation was carried out as described in Example
1, d). The results are set forth in FIG. 5. A current density of 3
A/cm.sup.2 was observed at 0.8 V dc operating voltage.
EXAMPLE 4
LSCFO-CGO-LSCFO/CGO (Asymmetric)
[0073] An electrochemical oxygen separator cell with the following
structure and composition was prepared and tested:
Cathode: Composition: LSCFO-80
[0074] Thickness: .about.20 .mu.m Electrolyte membrane:
Composition: CGO-20 [0075] Thickness: 8 .mu.m Anode: Composition:
30% wt. of CGO-20+70% wt of LSCFO-80 [0076] Thickness: .about.500
.mu.m.
[0077] CGO-20 powder was prepared as from Example 1, a).
[0078] The anode was prepared as from Example 3, a) starting from
0.21 g of CGO-20 and 0.49 g of LSCFO-80 (Praxair) which correspond
to a 30:70% wt.
[0079] The electrolyte membrane was prepared as from Example 3,
b).
[0080] The cathode was prepared as from Example 3, c) starting from
1 g of LSCFO-80 (Praxair).
[0081] The cell evaluation was carried out as described in Example
1, d). The results are set forth in FIG. 6. A current density of 6
A/cm.sup.2 was observed at 0.7 V dc operating voltage.
EXAMPLE 5
LSCFO-CGO-LSMO/CGO (Asymmetric)
[0082] An electrochemical oxygen separator cell with the following
structure and composition was prepared and tested:
Cathode: Composition: LSCFO-80
[0083] Thickness: .about.20 .mu.m. Electrolyte membrane:
Composition: CGO-20 [0084] Thickness: 8 .mu.m Anode: Composition:
30% wt. of CGO-20+70% wt of LSMO-80 [0085] Thickness: .about.500
.mu.m.
[0086] CGO-20 powder was prepared as from Example 1, a).
[0087] The anode was prepared as from Example 3, a) starting from
0.21 g of CGO-20 and 0.49 g of LSMO-80 (Praxair) which correspond
to a 30:70% wt.
[0088] The electrolyte membrane was prepared as from Example 3,
b).
[0089] The cathode was prepared as from Example 3, c) starting from
1 g of LSCFO-80 (Praxair).
[0090] The cell evaluation was carried out as described in Example
1, d). The results are set forth in FIG. 7. A current density of
1.5 A/cm.sup.2 was observed at 0.7 V dc operating voltage.
EXAMPLE 6
LSMO/CGO-CGO-LSCFO (Asymmetric)
[0091] An electrochemical oxygen separator cell with the following
structure and composition was prepared and tested:
Cathode: Composition: 30% wt. of CGO-20+70% wt of LSMO-80
[0092] Thickness: .about.500 .mu.m. Electrolyte membrane:
Composition: CGO-20 [0093] Thickness: 8 .mu.m Anode: Composition:
LSCFO-80 [0094] Thickness: .about.20 .mu.m.
[0095] CGO-20 powder was prepared as from Example 1, a).
[0096] The cathode was prepared as from Example 3, a) starting from
0.21 g of CGO-20 and 0.49 g of LSMO-80 (Praxair) which correspond
to a 30:70% wt.
[0097] The electrolyte membrane was prepared as from Example 3,
b).
[0098] The anode was prepared as from Example 3, c) starting from 1
g of LSCFO-80 (Praxair).
[0099] The cell evaluation was carried out as described in Example
1, d). The results are set forth in FIG. 8. A current density of
0.7 A/cm.sup.2 was observed at 0.7 V dc operating voltage.
[0100] The electrochemical oxygen separator cells of the invention
show a current density dramatically higher than that described in
the prior documents, obtained at the same voltage.
* * * * *